A number of processes exist in which a plastic sheet or film is formed on or against a release sheet and then separated from the release sheet after taking steps, such as cooling or curing, to set the sheet or film. Curing, where necessary, may be accomplished by heat, by peroxide catalyst, or U.V. radiation or by electron beam radiation. The release sheet provides a surface from which the set plastic material can be readily separated and imparts to the surface of the plastic material the quality of finish of the release surface. For example, a desired textured surface can be provided on the surface of the plastic material by forming on or against a release sheet having the reverse surface effect of the desired textured surface.
One example of such forming processes is "casting," wherein a resinous material, such as polyvinyl chloride or polyurethane resin, in a flowable state is deposited or "cast" onto the release sheet surface, heated, cured and cooled to consolidate the resinous material into a continuous self-supporting film, and stripped from the support. The release sheet is normally provided with a desired surface effect, such as high gloss, texturing or an embossed configuration, and the reverse of the surface effect is replicated on the cast film.
Another example of such forming processes is "panel pressing" of decorative plastic laminates, which can be either of the high pressure or low pressure type. In high pressure panel pressing, decorative laminates are conventionally prepared by assembling in a stacked relationship a plurality of core sheets, each of which is a web of paper impregnated with a resinous material, such as phenolic resin. Immediately positioned above the core sheet assembly is a decorative sheet, which is a resin saturated sheet having a solid color or a suitable design thereon. Superimposed above the decorative sheet is generally an overlay sheet which is a thin sheet of fine paper impregnated with a noble thermosetting resin, such as a melamine formaldehyde resin or an unsaturated polyester resin and the like (and is generally the same resin used to impregnate the decorative sheet). The entire assembly of core sheets, decorative sheet, and overlay sheet is placed between platens in a press and consolidated by application of heat and pressure. Generally, a release sheet having the desired surface effect to be reproduced in reverse in the surface of the overlay sheet is placed against the overlay sheet during pressing. High pressure laminates after being consolidated are usually further glued to a structural substrate, such as particle board or plywood. Low pressure panel pressed decorative laminates are made in a similar manner to high pressure laminates, but generally involve lamination of the decorative sheet directly to particle board or other structural substrate.
Other pressing processes where a plastic sheet or film is formed on or against a release sheet may not include the lamination step, but only texturing a moldable plastic surface which is already laminated. For example, a plastic film could be coated directly onto particle board or plywood and then textured by pressing against a release sheet having the desired textured pattern in its surface while setting the plastic film. (See, for example, U.S. Pat. No. 4,113,894 to Koch.)
Other uses for release sheets include heat transferable printed designs, pressure sensitive adhesive coated webs, and interleavers for panel pressing. The heat transferable printed designs are printed on the release sheet with a polyvinyl chloride plastisol ink or offset printing ink and overcoated with a polyvinyl chloride plastisol. When placed against a receptive surface, such as a T-shirt, and heated, the printed design and overlayer are transferred to the receptive surface where it sets upon cooling. In this application, the release surface must be capable of being printed upon and capable of releasing the printed material and overlying layer of polyvinyl chloride plastisol when it is transferred.
On the other hand, pressure sensitive coated webs are typically adhesive coated tapes, labels or decals and the like which are attached to a release surface for easy removal later when it is desired to permanently attach them to an object. The release surface must permit temporary attachment of the pressure sensitive adhesive, but also permit easy removal. The pressure sensitive adhesive laminates can be formed by coating a film of the adhesive onto the release surface and then attaching the backing sheet (i.e., tape), or the adhesive can be first coated on the backing sheet and then laminated to the release sheet.
An interleaver is a release sheet placed between groups of heat and pressure consolidated laminae pressed at the same time in back-to-back configuration to form two distinct decorative laminates. The release sheet in this case separates the laminates from each other and thereby permits more than one to be pressed at the same time between the same platens. (See, for example, U.S. Pat. No. 4,030,955 to Antonio et al.)
Release sheets are typically produced by coating, treating, or impregnating a paper web or other sheet-like substrate with a release coating of such materials as polymethylpentene, polypropylene, polyfluorocarbons, silicone oil, thermoset silicone resins, and other conventional release agents. Usually, release papers comprise a base sheet provided with at least one first coating, conventionally referred to as a base coating, and a release coating overlying the base coating. The base coating typically comprises a pigment or filler, such as a paper coating grade clay together with an adhesive binder. The purpose of the base coating is to level and seal the surface. The pigment in the coating provides a smooth surface by masking the irregularities in the surface of the base sheet, and the adhesive binds the pigment and prevents undue penetration into the base sheet by the subsequently applied release coating. The base coating thereby maximizes the effectiveness of the release coating applied by preventing excessive penetration of the release coating into the body-stock.
Surface effects on the release sheet, when desired, are conventionally provided by any one of a number of techniques. The release coating can be dried to a smooth surface gloss or surface effects such as texturing or embossing can be provided in the coating by mechanical means, applied either to the surface of the base paper before coating or to the paper after the release coating is applied. Another technique employed for producing a release coating with a textured surface is to extrude a molten thermoplastic film, such as polypropylene or polymethylpentene, onto a paper surface, cool it and then pass it between matched steel embossing rolls.